The present invention relates to a gas separation and purification process and an apparatus therefor, more specifically to a process and an apparatus for recovering a valuable gas in the form of purified product, and particularly to a gas separation and purification process and an apparatus therefor, which can be most suitably used for recovering and recycling valuable noble gases such as krypton and xenon to be used as atmospheric gases in semiconductor manufacturing equipment and the like.
In a process for manufacturing semiconductor products such as semiconductor integrated circuits, liquid crystal panels, solar panels and magnetic discs, there are used a wide variety of devices which generate plasma in a noble gas atmosphere to carry out various kinds of treatments for semiconductor products by the plasma thus generated, for example, sputtering machines, plasma CVD reactors, reactive ion etching machines, etc.
Such processing devices are operated as follows: When substrates and the like to be treated are fed into a processing chamber, a nitrogen gas atmosphere is formed in the processing chamber, and when a plasma treatment is carried out, a noble gas is charged singly or optionally together with a gas which promotes the reaction to the chamber to generate plasma by high-frequency discharge and carry out treatment of the substrates therewith. Subsequently, the chamber is purged by charging nitrogen gas thereto, and the substrates are taken out therefrom. As the gas for promoting the reaction in a treatment, for example, in a plasma oxidation treatment, a certain quantity of oxygen is added.
While argon has predominantly been used as the noble gas to be employed in such treatments, krypton and xenon having low ionization potentials are coming to the fore for more sophisticated applications.
Krypton and xenon are extremely expensive, since they are present in air at very low ratios and require intricate separation processes, so that the processes employing such gases are appreciated economically, only on the premise that used gases are recovered, purified and recycled.
A mixed gas containing a noble gas to be separated and purified consists mainly of a noble gas and nitrogen. In a plasma oxidation treatment, such a mixed gas containing additionally a certain quantity of oxygen is used. Meanwhile, in a plasma CVD treatment and a reactive ion etching treatment, a metal hydrogen compound gas and a halogenated carbon gas are additionally used respectively. Further, moisture, carbon monoxide, carbon dioxide, hydrogen, hydrocarbons, etc. are occasionally contained as trace impurities.
Xenon is also drawing attention for its application as an anesthetic gas in the form of mixture with a predetermined amount of oxygen (usually ca. 30%). The mixed gas to be subjected to the separation and purification treatment is a patient""s exhalation containing, for example, oxygen, nitrogen, carbon dioxide and moisture in addition to xenon. In this case, it is necessary to remove nitrogen, carbon dioxide, etc. from the mixed gas in order to recycle xenon.
Referring to the prior art to recover a specific component from a mixed gas by the pressure swing adsorption (PSA) process and purifying it, it is described extensively, for example, in a literature xe2x80x9cPressure Swing Adsorption, 1994 VCH Publishers Inc., collaborated by D. M. Ruthven, S. Farooq and K. S. Knaebelxe2x80x9d, Chapter 6.
Paragraph 6.5 of the literature explains a four-column PSA process for recovering hydrogen from various kinds of mixed gases and purifying it. This hydrogen PSA process utilizes the nature of hydrogen that it is extremely difficult to adsorb compared with other components of the mixed gas. Table 6.2 in the above literature shows test conditions and performance data of the four-column hydrogen PSA purification system. It is disclosed in Table 6.2 that if a high product (hydrogen) concentration of 99.9% or more is to be obtained in the conventional hydrogen PSA process, the rate of hydrogen recovery reduces to less than 80%.
Paragraph 6.6 of the above literature also explains a four-column PSA process for recovering carbon dioxide gas from a combustion waste gas and purifying it. Table 6.4 in the literature shows performance data of the PSA process for recovering carbon dioxide gas from a combustion waste gas and purifying it. It is disclosed in Table 6.4 that even if the product had a relatively low concentration of about 99%, the recovery rate of carbon dioxide gas was at most about 72%.
Paragraph 6.7 of the above literature also explains a PSA process for recovering methane from a gas occurring in dumpsite. It is disclosed in Paragraph 6.7 that when a recovery rate of 90% or higher is to be obtained by the conventional methane recovering PSA process, the product had a methane concentration of 89%.
Meanwhile, Japanese Unexamined Patent Publication No. H6-182133 (182133/94) discloses a process and an apparatus for recovering and purifying a noble gas in a high yield. This invention relates to recovery and purification of helium, and helium is recovered while an off-gas from the PSA process having been conventionally treated as a waste gas is recycled to be admixed to a raw gas, thus achieving high yield. However, the mixed raw gas is treated batchwise in this invention and cannot be treated continuously.
Japanese Unexamined Patent Publication No. H10-273307 (273307/98) discloses as follows: xe2x80x9cThe chamber is purged with a purge gas to form a gaseous outflow containing a noble gas and the purge gas, and the outflow is recovered from the chamber for recycling. The purge gas is preferably selected from hydrogen, steam, ammonia, carbon dioxide, carbon monoxide, oxygen and hydrocarbons having 2 to 6 carbon atoms. A noble gas flow is preferably separated from the outflow by means of membrane separation, condensation, adsorption, absorption, crystallization or by a combination thereof.xe2x80x9d
Further, Japanese Unexamined Patent Publication No. H11-157814 (157814/99) discloses a process relating to switching between an operation of introducing a noble gas-containing off-gas discharged from a plant where the noble gas is used to a recovery system and an operation of discharging it therefrom. However, this invention merely discloses as follows: xe2x80x9cWhile adsorption, membrane separation and the like can be employed, a getter type purification apparatus employing a metal such as titanium, vanadium, zirconium and nickel or an alloy thereof is suitably used.xe2x80x9d
Japanese Unexamined Patent Publication No 2000-171589 discloses a process for recovering krypton/xenon using natural zeolite as a process for recovering a radioactive noble gas. Although this invention discloses adsorption of the noble gas contained in helium gas, there is no disclosure of desorption, recovery nor recycling thereof.
Japanese Unexamined Patent Publication No. 2000-26319 discloses a process for recovering lower hydrocarbons from an off-gas from a polyolefin manufacturing plant by means of PSA process. This invention is directed to carrying out a recycling operation of mixing a purge off-gas to a raw gas so as to obtain a high recovery rate. However, according to embodiments of the invention, the recovery rate was about 90% when the lower hydrocarbon concentration was 99.9%, and 10% of lower hydrocarbons remained unrecovered.
As described heretofore, there has so far been neither process nor apparatus for recovering a specific component in a mixed gas by a PSA process continuously at a high purity and in a high recovery rate of 95 to 99% or more. Further, there are a very few published adsorption data on krypton and xenon. For example, Journal of Colloid and Interface Science, Vol. 29, No. 1, January 1969 describes adsorption data of krypton onto activated carbon and zeolite 5A at 25xc2x0 C. According to the data, it can be understood that activated carbon adsorbent adsorbs a large amount of krypton over zeolite 5A. A process for recovering an easily adsorbable component in the form of high-purity product is disclosed, for example, in Japanese Unexamined Patent Publication No. H3-12212 (12212/91), describing a process for separating and recovering nitrogen from air through three major steps of adsorption-cleaning-desorption.
In a consideration based on the adsorption data on krypton as described above, the activated carbon adsorbent has a sufficiently large amount of krypton adsorption compared with the adsorption of nitrogen as an impurity component, so that the process disclosed in Japanese Unexamined Patent Publication No. H3-12212 (12212/91) is deemed to be applicable. However, in the process ibid., since there is used a large amount of cleaning gas for obtaining a high-purity product, no high recovery rate can be expected.
That is, it has been difficult in the prior art to enhance sufficiently economical efficiency in the system handling a noble gas such as krypton and xenon. Particularly, none of the prior art techniques incorporated herein was successful in recovering a valuable noble gas by separation and purification from a mixed gas having been recovered by purging a chamber of a semiconductor manufacturing equipment with a nitrogen gas or by suction with a vacuum pump and containing the noble gas such as krypton and xenon like in the case as described above, in an amount of about 25 to 75%. Thus, a novel technology has been awaited to be exploited.
As described above, in the conventional semiconductor manufacturing equipment and the like, once a valuable gas such as krypton or xenon is used as an atmospheric gas, it is released to the outside, so that the cost of atmospheric gas notably increases, disadvantageously. Besides, there is a problem that it has been difficult to recover the valuable noble gas from the chamber of the semiconductor manufacturing equipment continuously by means of PSA process at a high purity and in a high recovery rate of 95% or more, and it has been far more difficult technologically to give a higher recovery rate of 99% or more.
Therefore, the present invention is directed to providing a gas separation and purification process and an apparatus therefor, which can recover a valuable gas and purifying it efficiently by means of PSA process using as a raw gas a mixed gas containing a valuable gas such as krypton and xenon to be used as an atmospheric gas in semiconductor manufacturing equipment and the like.
In the gas separation and purification process according to the present invention, a valuable gas is separated in the form of purified product from a mixed gas, used as a raw gas, containing the valuable gas by means of pressure swing adsorption process, wherein the pressure swing adsorption process contains a combination of equilibrium pressure swing adsorption process for separating gas components based on the difference in the amounts of adsorbed gases at equilibrium, and rate-dependent pressure swing process for separating the gas components based on the difference in adsorption rates.
In the gas separation and purification process according to the present invention, the raw gas is separated into an easily adsorbable component and a hardly adsorbable component by the equilibrium pressure swing adsorption process to release the hardly adsorbable component in the equilibrium pressure swing adsorption process as an off-gas, whereas the easily adsorbable component in the equilibrium pressure swing adsorption process is separated into an easily adsorbable component and a hardly adsorbable component by the rate-dependent pressure swing adsorption process to collect the hardly adsorbable component in the rate-dependent pressure swing adsorption process as a product gas. Particularly, the easily adsorbable component in the rate-dependent pressure swing adsorption process is circulated to the equilibrium pressure swing adsorption process to be subjected to re-separation there.
Further, the raw gas is separated into an easily adsorbable component and a hardly adsorbable component by the rate-dependent pressure swing adsorption process to collect the hardly adsorbable component in the rate-dependent pressure swing adsorption process as a product gas, whereas the easily adsorbable component in the rate-dependent pressure swing adsorption process is separated into an easily adsorbable component and a hardly adsorbable component by the equilibrium pressure swing adsorption process to release the hardly adsorbable component in the equilibrium pressure swing adsorption process as an off-gas. Particularly, the easily adsorbable component in the equilibrium pressure swing adsorption process is circulated to the rate-dependent pressure swing adsorption process to be subjected to re-separation there.
Further, the raw gas is separated into an easily adsorbable component and a hardly adsorbable component by the rate-dependent pressure swing adsorption process to release the easily adsorbable component in the rate-dependent pressure swing adsorption process as an off-gas, whereas the hardly adsorbable component in the rate-dependent pressure swing adsorption process is separated into an easily adsorbable component and a hardly adsorbable component by the equilibrium pressure swing adsorption process to collect the easily adsorbable component in the equilibrium pressure swing adsorption process as a product gas. Particularly, the hardly adsorbable component in the equilibrium pressure swing adsorption process is circulated to the rate-dependent pressure swing adsorption process to be subjected to re-separation there.
Further, the raw gas is partly separated into an easily adsorbable component and a hardly adsorbable component by the equilibrium pressure swing adsorption process to release the hardly adsorbable component in the equilibrium pressure swing adsorption process as an off-gas, and the easily adsorbable component in the equilibrium pressure swing adsorption process is admixed with the raw gas; whereas the rest of the raw gas is separated into an easily adsorbable component and a hardly adsorbable component by the rate-dependent pressure swing adsorption process to collect the hardly adsorbable component in the rate-dependent pressure swing adsorption process as a product gas, and the easily adsorbable component in the rate-dependent pressure swing adsorption process is admixed with the raw gas. Particularly, the raw gas is supplied to the equilibrium pressure swing adsorption process and the rate-dependent pressure swing adsorption process after pressurization, and the easily adsorbable component in the equilibrium pressure swing adsorption process and the easily adsorbable component in the rate-dependent pressure swing adsorption process are admixed with the raw gas before pressurization.
In a gas separation and purification apparatus according to the present invention, a valuable gas is separated in the form of purified product from a mixed gas, used as a raw gas, containing the valuable gas by a pressure swing adsorption unit, wherein the pressure swing adsorption unit contains an equilibrium pressure swing adsorption unit, which separates gas components based on the difference in equilibrium adsorption, and a rate-dependent pressure swing adsorption unit, which separates the gas components based on the difference in adsorption rates.
Further, in the gas separation and purification apparatus according to the present invention, the equilibrium pressure swing adsorption unit is connected in series with the rate-dependent pressure swing adsorption unit such that the former unit is located on the upstream side and the latter unit is located on the downstream side; the upstream equilibrium pressure swing adsorption unit is provided with a passage for extracting a hardly adsorbable component in the equilibrium pressure swing adsorption unit as an off-gas and a passage for introducing an easily adsorbable component in the equilibrium pressure swing adsorption unit to the downstream rate-dependent pressure swing adsorption unit; whereas the downstream rate-dependent pressure swing adsorption unit is provided with a passage for extracting a hardly adsorbable component in the rate-dependent pressure swing adsorption unit as a product gas and a passage for circulating an easily adsorbable component in the rate-dependent pressure swing adsorption unit to the raw gas supply side of the equilibrium pressure swing adsorption unit.
Further, the gas separation and purification apparatus is further provided with a passage for diverting the raw gas and supplying it into the equilibrium pressure swing adsorption unit and to the downstream rate-dependent pressure swing adsorption unit; a passage for admixing the easily adsorbable component in the equilibrium pressure swing adsorption unit with the raw gas to be supplied to the rate-dependent pressure swing adsorption unit; and a passage for admixing the easily adsorbable component in the rate-dependent pressure swing adsorption unit with the raw gas to be supplied to the equilibrium pressure swing adsorption unit.
Further, the equilibrium pressure swing adsorption unit uses activated carbon as an adsorbent. The rate-dependent pressure swing adsorption unit uses Naxe2x80x94A zeolite or carbon molecular sieve as an adsorbent. The valuable gas is at least one of krypton and xenon.
According to the present invention, since valuable gases such as krypton and xenon contained in a mixed gas to be discharged from a semiconductor manufacturing equipment and the like using such a valuable gas can be recovered at a high purity and in a high recovery rate, the cost of the atmospheric gas in the semiconductor manufacturing equipment etc. can be reduced notably.